Thermally-responsive starting device for a single-phase asynchronous motor



Feb. 10, 1970 A. F. ENEMARK 3, 9

THERMALLY-RESPONSIVE STARTING DEVICE FOR A Q SINGLE-PHASE ASYNCHRONOUS MOTOR Filed Aug. 25. 1967 Ami-5% FIG.

United States Patent M US. Cl. 318221 13 Claims ABSTRACT OF THE DISCLOSURE A thermally-responsive starting device for starting single-phase asynchronous induction motors having a main winding for driving the motor and a starting winding in a split-phase configuration. The starting device is a switch having normally closed contacts and at least one bimetallic strip carrying a contact and opening the switch contacts to take the motor starting winding out of circuit at a selected time or speed of the motor under con trol of two heat sources, namely two resistors. One of the resistors is in series with the starting winding and When the motor is started this resistor heats the bimetallic element which opens the starting switch contacts taking the starting winding out of circuit. The opening of the switch deenergizes the series resistor and energizes a second or holding resistor in parallel with the switch for holding the switch open by heating the bimetallic element. The lag in heating the second resistor is offset by the series resistor which comprises a material dimensioned to cause the series resistor to act as a heat-storing device continuing to heat the bimetallic strip until the holding resistor is fully eifective. Thus the switch time of opening can be readily controlled and there is no possibility of the bimetallic strip cooling sufficiently during the lag period and making undesirable intermittent clos ings of the starting device.

This invention relates generally to motor starting devices and more particularly to a thermally-responsive starting device for a single-phase asynchronous motor in which the device comprises a bimetallic switch, in the circuit of the starting winding, which opens under the influence of one heat source, an opening source which is traversed by the switch current, and is then kept open by another heat source, a holding source.

A motor starting device of this type is known in which a bimetallic element or strip itself constitutes the opening source and, when traversed during the starting period by the current flowing through the starting winding, is heated sufiiciently for the switch to open. The holding source is a second bimetallic strip which, so long as the motor keeps running, is supplied with a heating current as a result of the voltage applied to an auxiliary winding and keeps the switch open even after the firstmentioned bimetallic strip has cooled down.

A thermally-responsive motor starting device has also been proposed in which the holding source consists of a heating resistor to which is applied a vectorial potential difference between the voltage across the running winding and the voltage across the starting winding of the motor and which, preferably, shunts the bimetallic switch directly. This type of starting device is of extremely simple construction and operates very safely. However, it is necessary to ensure that the full eifect of the holding resistor acts upon the bimetallic switch immediately after it has opened, otherwise the switch can assume its closed position.

3,495,150 Patented Feb. 10, 1970 A principal object of the present invention is, therefore, to provide a thermally-responsive starting device for a single-phase asynchronous motor which, on the one hand, is of a very simple construction and operationally very reliable and, on the other hand, will not execute repeated on and off switching at the time of interrupting the starting winding circuit.

Another object of the present invention is to provide a motor starting device especially suitable for motors with a high starting torque and also makes possible other advantageous features.

The objects are achieved by the invention in that the holding source comprises two thermally cooperative heating elem nts one of which is capable of transmitting to the switch the heat necessary to keep it open only when a certain period of time has elapsed following the instant of its opening and interrupting the starting winding current while the other supplies the switch with the heat needed for keeping it open at least during this period.

The first-mentioned heating element, which must remain operative all the time while the motor runs, may thus be any type of heater regarding its shape, position and material. In particular, it is possible to employ in starting devices of this type a bimetallic switch and to let it become operative, i.e., to supply to it an appreciable current only when the oflF-period commences without having to make sure, at the same time, that a suflicient amount of heat is immediately transmitted to the switch in order to keep it open. Consequently, the thermal conductivity of the transmission path between heating device and the bimetallic elements is only of secondary importance. It is also possible to use resistor elements for the heater which consist of a material warming up slowly. Such a resistor element is of particular advantage at the lower end of the bimetallic strip opening and closing the switch or device.

On the other hand, the use of a second heating element forming part of the thermally-responsive starting device does not complicate the construction of the device to any appreciable extent. All that is required of it is the supply of a supplementary amount of heat during a comparatively short period of time immediately after the instant the starting winding circuit is opened. This can be accomplished in a very simple manner, for example by means of a heat-storing device included in the circuit of the switch.

In a preferred embodiment of the invention the second heating element is a heat storage device and the switch, when opening, renders the first heating element operative and the seocnd heating element inoperative. A very simple circuit arrangement is obtained if the first heating element is a resistor connected in parallel with a contact bimetallic strip in the switch and the second heating element is a resistor connected in series with the bimetallic strip, and if the second heating element has a slower thermal response than the first heating element.

The second heating element may be constructed as a resistor in series with the contact bimetallic strip and corresponds to an opening source. The advantage of such an arrangement is not only a simplification of the construction, but also the fact that the opening source is formed by a slow-response heating element. In this way the switch-on time of the device is prolonged without any other measure having to be effected. Thus, a voltage continuous to be applied across the auxiliary or starting Winding for a longer period of time and even electric motors with a high starting torque can be started in this way. Conversely, slow starting becomes an acceptable feature so that cheaper motors become possible.

It has been found that the time constant of the second heating element should be at least twice as large as that of the first heating element. In the present invention the time constant is related not only to the heating element itself, but also to the entire system comprising the transmission paths and the bimetallic strip in any given case. The larger the difference between the time constants, the more safely will the switch-01f temperature of the bimetallic element be maintained.

For a single-phase motor with a resistive auxiliary phase it is recommended that the resistance value of the seriesconnected heating element be at least 10% and preferably 50% of the total resistance of the auxiliary or starting phase. Thus, the series resistor acting as a heat-storing deivce assumes also the function of a starting resistor so that the generation of heat in the motor itself is reduced. Furthermore, this series-connected heating element is capable of absorbing an appreciable amount of power, since it can be of very rugged design. Conversely, the capability of absorbing a suitable amount of power makes possible the delivery of an appreciable amount of heat for a considerable time after the switch-off.

The series-connected heating element preferably consists of carbon (graphite) or of silicon carbide. Resistors can be produced with these materials which may be loaded with a few hundred watts or more and which have a high heat storage capacity, due to their mass, unlike a wire which has very little heat storage potential.

In a preferred embodiment the parallel-connected heating element is a PTO-resistor. A resistor of this type heats up ripidly after a voltage has been applied to it, so that its thermal response is very fast. This makes it possible to use a faster element also for the series-connected heating resistor. Moreover, its delivery of power is largely independent from the mains voltage.

A simple construction can be obtained if the seriesconnected heating element supports a fixed bimetallic strip having a switch contact. The parallel-connected heating aresistor may then be directly attached to the series-com nected heating resistor at the end opposite this contact and may support the bimetallic strip at the end away from the series-resistor. This designcomprises only the bare electrical essentials, is small and compact and guarantees effective transfer of heat due to the parallel position of the bimetallic strip and the series-connected heating resistor.

Other features and advantages of the motor starting device in accordance with the present invention will be better understood as described in the following specification and appended claims, in conjunction with the following drawings in which:

FIG. 1, is a diagrammatic illustration of a first embodi ment of the invention included in a starting circuit of a split-phase induction motor; and

FIG. 2, is a fragmentary diagram of a second embodiment of starting switch according to the invention.

In FIG. 1 a single-phase asynchronous motor 1 comprising a main winding 2 and an auxiliary or starting winding 3 is connected to supply mains terminals 5 by means of a main switch 4. A thermally-responsive starting device 6 according to the invention is connected in the circuit of the starting winding.

The starting device 6 comprises a resistive rod 7 of silicon carbide to the upper end of which is soldered a fixed contact 8. At the lower end of the rod is soldered an annular PTC-resistor 9. A free end of the resistor 9 has attached thereto a bimetallic strip 10' by means of a rivet 11 with an insulating washer 12 insulating the rivet head from the annular resistor. The bimetallic strip carries the other or free end a movable switch contact 13. The leads to the motor circuit 1 are connected at points 14 and 15. The resistive rod or pile 7 is a heating resistor connected in series with the switch and the conductive bimetallic strip 10 while the ring or annulus 9 is a heating resistor in parallel with the bimetallic strip and the switch. Due to its greater mass 7, the thermal response of the series resistor is slower than that of the parallel resistor 9.

When the main switch 4 is closed a voltage is applied across the motors main winding 2- directly, and across the auxiliary or starting winding 3 through the starting device 6. The current flowing through the starting winding heats the series-connected resistor 7 and it radiates heat to the bimetallic strip 10 until the contacts 8, 13, which are normally closed before the motor starts, open. While the switch is closed the current flowing through the parallel resistor 9 is negligible so that it does not heat up. As soon as the bimetallic strip heats and opens the switch, however, the voltage applied across the parallel resistor rises so that the latter heats up. However, a certain period of time elapses until the resistor element 9 heats up and its 'heat is transmitted effectively to the bimetallic strip. Nevertheless, the bimetallic strip 10 will not return to its normal switch-closing position because it continues to be heated by the resistive pile or rod 7. Although no more power is supplied to this resistive element after the contact pair 8, 13 has opened, the heat stored in it is suflicient to prevent the bimetallic strip from closing the switch contacts.

In a practical embodiment of the invention the starting device was dimensioned as follows: the series resistor 7 and the starting winding 3 has identical resistance values, i.e., 30 ohms. The parallel resistor '9 was a PTC- resistor which, at room temperature, had a value of 300 ohms and, after attaining equilibrium temperature, a value of 30 kilo-ohms. The bimetallic switch opened after ten seconds, so that it was suitable even for motors with a high starting torque. Power consumption during the running of the motor was accordingly low.

In the embodiment of FIG. 2 the series resistor has a shape of a pile 16 and is arranged between two bimetallic strips 17 and 18 each of which supports a movable contact, 19 and 20, respectively. The strip 17 has its lower end directly connected to the series resistor 16 in an electrically conductive manner and the bimetallic strip 18 is connected at its lower end to the series resistor 16 through a parallel resistor 21. The leads connecting the device to the supply mains and motor, not shown, are connected to a point 22 at the upper end of the series resistor 16 and a point 23 at the lower end of the bimetallic strip 18. In this embodiment the heat radiation of the series resistor is exploited to even greater advantage. Moreover, the parallel resistor 21 directly shunts the contacts 19, 20, whereas in the embodiment of FIG. 1 the series circuit comprising the series resistor 7 and the contacts 8, 13 has to be shunted.

A great many modifications in the constructions abovedescribed are possible without constituting a departure from the basic concept of the invention. If the series resistor 7 is to absorb a large amount of power it may be made with an annular shape with the switch contact 8 and the parallel resistor 9 arranged at diametrically opposite points. Soldered connections may be replaced by rivets or other fastening means. The series resistor need not be in a thermally conductive connection with the bimetallic strip 10; it may also act upon the bimetallic strip by radiating heat.

While preferred embodiments of the invention have been shown and described it will be understood that many modifications and changes can be made within the true spirit and scope of the invention.

What I claim and desire to be secured by Letters Patent 1s:

1. A starting device for a single-phase asynchronous motor having a main winding and a starting winding in a split-phase configuration, said starting device comprising switch means normally closed having electrically conductive thermally-responsive means connectable with said starting winding for taking said starting winding out of circuit after said motor starts in response to current flow through said starting winding, 'an electrically energized source of heat connected in use with said thermallyresponsive means, said thermally-responsive means being disposed to receive heat from said source of heat for opening said switch means and taking said starting winding out of circuit after said motor starts, another electrically energized source of heat connected in said starting device for holding said thermally-responsive means heated holding said switch means open and maintaining said starting winding out of circuit, said another source of heat connected in said starting device to be rendered effective when said thermally-responsive means opens said switch means and electrically de-energizes the firstmentioned source of heat, and said first-mentioned source of heat comprising a material effective to land dimensioned to retain suflicient heat when electrically deenergized to maintain said thermally-responsive means heated holding said switch means open during the interval in which said another source of heat generates sufi'lcient heat to effectively heat said thermally-responsive means without assistance from said first-mentioned source of heat and thereby to maintain said switch means open without said thermally-responsive means cooling and closing said switch means during the interval in which said another heat source heats up.

2. Thermally-responsive starting device for a single phase asynchronous motor having a main winding circuit and a starting winding circuit said starting device having a first heat source for opening said starting winding circuit and a second heat source and a bimetallic switch contact element in the circuit of the starting winding opening the circuit of said starting winding under the influence of said first heat source and connected to be traversed by the current in said starting winding and then kept open by said second heat source comprising a holding source, said holding source comprising a first heating element capable of transmitting to the bimetallic switch contact element the heat necessary for keeping it open only after a certain period of time has elapsed following opening of the starting winding circuit and a second heating element corresponding to said first heat source and delivering heat necessary to keep the bimetallic switch contact element open at least during that period.

3. A thermally-responsive starting device according to claim 2, in which said second heating element is a heatstoring device, and electrical connections connected so that the bimetallic switch contact element on opening the starting winding circuit renders the first heating element effective and the secondheating element ineffective.

4. A thermally-responsive starting device according to claim 2, in which the first heating element is a heating resistor connected in parallel with the bimetallic switch contact element and the second heating element is a heating resistor connected in series with the bimetallic switch 6 contact element and said second heating resistor has a slower thermal response than the first heating resistor.

5. A thermally-responsive starting device according to claim 4, in which the resistance value of the seriesconnected heating resistor is at least 10% and in the order of 50%, of the total resistance of the starting winding circuit. I

6. A thermally-responsive starting device according to claim 4, in which the series-connected heating resistor consists of graphite.

7. A thermally-responsive starting device according to claim 4, in which the series-connected heating resistor consists of silcon carbide.

8. A thermally-responsive starting device according to claim 4, in which the parallel-connected heating resistor is a FTC-resistor.

9. A thermally-responsive starting device according to claim 4, in which the series-connected heating resistor is a fixed contact of the starting device.

10. A thermally-responsive starting device according to claim 9, in which the parallel-connected heating resistor is directly attached to the series-connected heating resistor at an end opposite the contact and supports said bimetalic switch contact element at an end away from .the series-connected heating resistor.

11. A theremally-responsive starting device according to claim 2, in which said second heating element is a heating resistor corresponding to an open source for said starting device and is connected in series with the bimetallic switch contact element.

12. A thermally-responsive starting device according to claim 11, in which the second heating element is attached to the lowest point of the bimetallic switch contact element.

13. A thermally-responsive starting device according to claim 2, in which the time constant of the second heating element is at least twice as large as that of the first heating element.

References Cited UNITED STATES PATENTS 2,417,912 3/1947 Clark 318221.9 XR 2,618,721 11/1952 Clark 318 221.9 XR

FOREIGN PATENTS 392,490 5/1933 Great Britain.

ORIS L. RADER, Primary Examiner GENE RUBINSON, Asistant Examiner U.S. C1. X.R. 318229 

